A wastewater mixture of solutes and water solvent will freeze at lower and lower temperatures with corresponding increased concentration of solutes. Higher concentrations of salt require colder and colder temperatures for ice (pure water in frozen state) to separate from the saltwater solution (brine). As the temperature reduces further, more ice separates and the remaining liquid contains the same amount of salt but less water, thus the remaining solution becomes more concentrated. The solid ice is of lower density and the liquid brine is of higher density so the solid ice floats to the top of the liquid brine.
When the temperature is further reduced, liquid brine reaches a saturation solution of salt in the brine. Salt starts to form from within the brine. Since the salt crystals are denser than the saturated brine, the salt crystals deposit at the bottom of the saturated brine solution. This is the eutectic point.
Depending on the temperature setting, the following salts are produced in the process: −1.3 degrees Celsius (° C.) produces Glauber's salt, used in washing powders and detergents; −2.0° C., produces gypsum, used for buildings, walls, plaster of Paris; −3.9° C. produces Epsom salt; and −21.12° C. produces table salt.
Large quantities of polluted water are inevitable with mining, but a pioneering freeze crystallization waste-water process, which began operating Aug. 10, 2017 after 10 years of research, has been hailed as a revolutionary step in addressing South Africa's water crisis.
The hi-tech system, which uses a specialized freezing process to extract clean water from the brine byproduct of desalinated mining water, has begun operating at Glencore's Tweefontein coal mine in eMalahleni, Mpumalanga, South Africa.
When fully operational, it will produce 500,000 liters of potable water a day, most of which is due to be sold to the local municipality for use by residents. It is believed to be the first time in the world that this technology has been implemented on such a large scale, and, if rolled out to similar mines in South Africa, it could produce billions of liters of additional potable water daily.
Some 95% of polluted coal mining water can be converted to potable water through desalination. The remaining 5% is brine, which until now has been pumped into large pools so it can evaporate. This process is expensive, time-consuming and not completely effective. That 5% equates to 500,000 liters of clean water a day once it has gone through the hi-tech freezing process.
Research for the system, called eutectic freeze crystallization, was funded by the Water Research Commission and the Coaltech Research Association, a collaborative initiative between the Chamber of Mines, several mining companies and the universities of the Witwatersrand, Johannesburg and Pretoria.
Jo Burgess, of the Water Research Commission, said: “If we could have [this system] at every mine which currently has brine ponds, we could recover billions of liters of water and use it for domestic and industrial water supplies. We could alleviate the drought instead of wasting all that evaporated water—millions of liters each and every day. South Africa's water supply would be safer and more secure.”
The elements required to successfully design a compact, portable, lightweight freeze crystallization spray chamber or a eutectic freeze crystallization spray chamber are: time to freeze a wastewater droplet to form an outer shell of frozen fresh water; time for ice shell to fracture and release interior dense liquid droplet of high concentration wastewater; separation of fresh water ice from mound of ice that has accumulated at the bottom of the chamber by draining dense and high concentration liquid from each ice crystal surface; and further separation of fresh water ice from mound of ice by washing remaining layer of liquid waste water from each surface of ice.
Gerald Stepakoff and David Siegelman presented, “Application of a Eutectic Freezing System to Industrial Waste Water Recycling” at the Industrial Waste Water Reuse Conference, Environmental Section, American Institute of Chemical Engineers in Washington, D.C. on 24 on April 1973, as part of the Avco Water Purification Program of the Avco Systems Division, Wilmington, Mass. The freezing was carried out by direct contact heat transfer, by evaporation of an immiscible refrigerant from the waste water in a closed vessel. The process uses Freon refrigerants (nonflammable and non-toxic), direct contact heat transfer, stirred tank or spray freezers, cyclone separators, and a pressurized counterwasher.
The eutectic freezing process has application to both brackish water and industrial waste water treatment. In the case of brackish waters, many inland desalting plants operate to recover from 50 to 80% of the feed water as fresh water. However, a concentrated waste brine stream is also produced along with the product water, and the volume of this waste may comprise 20% to 50% of the feed volume flow. This presents a formidable disposal problem. For the latter case, many industrial wastes contain dissolved solids which have economic value if recovered and returned to the main process. Also, in many cases, the waste waters cannot be disposed of because of the toxicity of the dissolved chemicals. Freezing or eutectic freezing involving two-stage freeze concentration, can be incorporated in such cases into a recycling process for water reuse of industrial and chemical wastes industrial and chemical wastes.
There are several general advantages of freezing for purifying waste water as well as specific advantages of the freezing or eutectic freezing process for recovery of dissolved solids. In general, freezing processes have a low specific energy requirement. For example, with a 3.5% saline sea water feed the specific energy is about 45 to 50 kwhr/1000 gallons of product for a 1 mgpd (1 million gallons per day) plant. Freezing processes based on a secondary refrigerant process have low capital cost and there is virtually no corrosion because of the low temperatures.
The freezing process is also nearly universal in that there are no restrictions on the feed composition or chemical makeup of the waste water. Eutectic freezing, which is the companion process to the secondary refrigerant process, has the advantage of achieving very high concentration factors with a large reduction in waste volume. In many cases, it is possible to produce a solid waste which is easier to dispose of than a liquid. The process can recover valuable solids without corrosion. It has lower power consumption than evaporation. There are no geological handicaps, as with deep well disposal. There are no climatic restrictions or large land area requirements, as with solar evaporation. Finally, eutectic freezing plants can be used in conjunction with waste brine from any type of primary processing plant using Reverse Osmosis, Electrodialysis, evaporation, or freezing. This freezing is carried out by direct contact heat transfer by evaporation of an immiscible refrigerant, from the waste water in a closed vessel. However, it would be beneficial to carry out freezing at much colder temperatures.
Wa Gao's doctoral thesis at the University of Alberta is entitled, “Partial Freezing by Spraying as a Treatment Alternative of Selected Industrial Wastes”, 1998. The principle objective of this study was to evaluate the spray freezing process as treatment alternative for industrial wastewater. It included the investigation of the ice nucleation characteristics of pulp mill effluent, piggery wastewater and oil sands Tailings Pond Water (T.P.W) droplets, the freezing behavior of freely suspended wastewater droplets and impurity rejection and concentration phenomena occurred in the freezing and melting process.
The laboratory experiments showed that wastewater droplets made from different wastewater froze at different temperatures when they were tested under the same experimental conditions. When a water drop was freely suspended in an updraft of cold air, the freezing started at the bottom of the drop and then spread over the entire surface enveloping the drop in an ice shell. The freezing temperature of a droplet was influenced by the nature of the wastewater, the ambient air temperature, the droplet size, the impurity concentration and the pH of the wastewater.
When wastewater was sprayed into a cold atmosphere, the contaminants in the wastewater were rejected by the growing ice crystals and were concentrated in the liquid phase as part of the sprayed water froze. The unfrozen water generated in the spray freezing process could carry away more than 50% of the impurities in the source water from the ice mound. The spray ice impurity concentration could be predicted by a mathematical model based on the mass balance of the impurity in the continuous spray freezing process.
After a wastewater drop was introduced in the updraft, it remained liquid as it was super-cooled. The larger water drop suspending in air did not maintain a “spherical” shape: it exhibited a marked flattening on its lower surface and smoothly rounded curvature on its upper surfaces. The deformation increased as the droplet volume increased.
The video recording reveals that freezing starts at the bottom edge of a water droplet and then envelopes the whole surface area of the top progressively at all ambient temperatures used for this study. Then, it freezes inward and the ice shell thickens as phase change continues.
Plate 4.1 shows the freezing process of a 2.8 mm diameter pulp mill effluent drop. The ambient air temperature was −5.5° C. and humidity 80.1%. The droplet was introduced into the updraft of the wind tunnel.
After 1/30 second, freezing started at the bottom edge of the droplet. The freezing proceeded to the entire surface of the droplet. The droplet surface freezing was completed in 7/30 second.
The time required to envelop the entire drop surface area of different drops varied. It depended on the freezing temperature (or cooling rate) and the type of the water. Under the same conditions, tap water or distilled water (with food color) drops, only needed 2/30 to 3/30 second to finish the surface freezing. But it took more time to envelop the surface of wastewater drops (3 to 4 frames or even longer) although the time required to initiate the freezing by the wastewater drops was much shorter.
Plate 4.2 (Gao Thesis) displays the freezing of a 3.4 mm diameter piggery wastewater drop in a −17.7° C. environment and the pH of the droplet was adjusted to 11.0. It again took 7/30 second, while it only took 4/30 second for a 4.2 mm tap water droplet to finish the surface freezing. It was even shorter, 2/30 seconds, for the ice formation on the surface of a 4.2 mm distilled water drop to which red food coloring had been added.
Blanchard (1955) observed freezing of large water drops suspended in a vertical wind tunnel. In his work Blanchard showed that the manner of freezing is a function of drop temperature. Blanchard indicated that −5° C. was the approximate dividing temperature between clear ice which forms at warmer temperature and opaque ice which forms at colder temperature.
The phenomena of fragmentation of freezing water drops was observed by many researchers (e.g. Mason, 1965a: Langham and Mason, 1958: Mason and Maybank. 1960: Dye and Hobbs. 1968; Hallett. 1968 and Hobbs and Alkenzweeny. 1968). It also was observed in the Blanchard (1955) study that as freezing proceeded the ice shell of some drops fractured and the unfrozen liquid inside of the drop squeezed out on to the ice surface of the drop.
The crack usually occurs at the top of the drop. Then a protrusion formed as the top continued to remain suspended in the updraft. Among the wastewaters tested, formation of protrusion only occurred in pulp mill effluent and Tail Pond Water (T.P.W.) and no bulging ever occurred in the piggery wastewater drops. The impurities will be concentrated in the unfrozen liquid and then is squeezed out of the drop.
Fracture of the ice shell of frozen drops occurs in the second stage of freezing. During this stage of freezing, as mentioned before, ice continues to grow rapidly inwards from the surface of the drop. Expansion caused by phase change of the water inside of the drop during this stage builds up pressure in the interior of the drop. Rupture of the shell, occurs at a weak point where a protrusion forms as the liquid from the interior is extruded,
The probability for a drop to fracture or fragment during freezing is affected by many factors. It is known that the nucleation temperature of the drop is one of them. The probability of fragmentation is higher for a drop with higher nucleation temperature since there is a larger amount of water remaining to undergo phase change in the second stage of the freezing.
Mason and Maybank (1960) indicated that the air content of the water influences the shattering of freezing water drops. The air content of a drop is controlled by the drop temperature since the solubility of air in water increases rapidly with decreasing temperature. When a drop nucleates at a warmer temperature, a very small amount of air can escape from the surface to the atmosphere and the ice shell formed is mechanically strong. A protrusion may appear at the weak spot and develop into a spike. A drop may even break into several fragments when the expansion of the interior cannot be held by extrusion through spikes and through the nearly impervious shell. Larger quantities of air would be liberated and trapped in the ice shell and cause a spongy texture in it when a drop freezes under strong super-cooling. This yields more readily to the expansion, part of which is taken up by compression of the entrapped air. Numerous cracks and fissures appear in the ice shell through which the liquid is exuded. The occurrence of spikes and violent shattering is rare and fewer ice splinters are produced. More recent tests (Sander Wildeman, February, 2017) using a 25 million frame per second camera showed rapid brittle fracture of the ice shell.
The low nucleation temperature of piggery wastewater drops may be one of the reasons that prevent piggery wastewater drops from breaking during freezing. The nucleation temperature of piggery wastewater is several degrees colder than that of pulp mill effluent and oil sands tailing pond water.
It also explains why smaller drops are less likely to fragment: drops with smaller diameter have lower nucleation temperature. Only cracking of frozen drops were observed in this study and no shattering of drops occurred.
Dye and Hobbs (1968) pointed out that except for the nucleation temperature of the drop, the nature and concentration of gases dissolved in the drop prior to its nucleation, the condition of the drop with respect to its environment and the manner in which heat is removed from a drop will affect the freezing behavior of a suspended drop.
Dye and Hobbs (1968) found that if drops which are nucleated before coming to thermal equilibrium with the environment, or are nucleated at warmer temperatures and then freeze rapidly at a lower temperature will show more disruptive activity during freezing than do drops frozen at thermal equilibrium. A fast freezing rate and symmetrical heat transfer are likely to favor fragmentation. Since the ice shell has less time to accommodate to the rapidly increasing pressure at fast freezing rate, the chances for a frozen drop to break may increase. If the ice shell freezes asymmetrically, it will not have a uniform strength so the pressure may be relieved at a weak point in the shell by forming a protrusion or a spike without shattering of the drop while the pressure may only be relieved by breaking the drop when an ice shell grows symmetrically with respect to the center of the drop.
The spray freezing frozen mass is very porous. The density of the mass is approximately 0.5 to 0.6 g/cc, or almost half that of pure, high quality ice. To look at it, it looks more like snow than ice. The brine therefore drains down through a porous network of voids that is established and maintained as the pile of spray ice forms. As the spray freezing pile grows, there is a constant flow of drained brine out of the base of the pile. Keep in mind that the temperature in the spray ice pile is essentially 0° C. as long as spraying is ongoing, due to the large amount of heat associated with the phase change.
The final product of the freeze crystallization process is very much dependent on the dissolved species in the water. In work at the Colomac Mine, the water contained arsenic and thiocyanates at low concentrations, so was not considered potable. If the only contaminants are say road salts, the final product should have sufficiently low concentrations. However, one must also measure for other dissolved species. The quality of the water also depends on the rate of thawing. Natural thawing under ambient temperatures is generally slow so is more efficient. Higher thawing gradients result in less efficient solute removal.
The time to freeze a droplet is taken from the PhD thesis of Kassem Al-Hakim. “An investigation of spray-freezing and spray-freeze-dryings”, 2004. Pages 252 to 254 use freezing temperatures: −25° C.-45° C. and −65° C. or −5.8 degrees Fahrenheit (° F.), −13° F. and −49° F., respectively.
Important data to be taken from Al-Hakim includes that freezing times are increased with increase in droplet diameter, and freezing times are shortened by colder ambient gas temperature. The estimated freezing temperature vs. time profiles shown in Figure (5-56) indicate that a 20 micron drop will freeze in 14 milliseconds at −20° C. or −4° F., depending on the relative velocity profiling of the drop . . . and at 7 milliseconds at −65° C.=−85° F.
Droplets of water can burst apart when they freeze, sending out shards of ice in all directions. Sander Wildeman at the University of Twente in the Netherlands and colleagues have now filmed this process in unprecedented detail—from the formation of the first ice crystal to the final bang. This footage, plus the groups' model-based calculations, reveals when and why water drops rupture as they freeze from the outside in.
The team began by super-cooling a roughly millimeter-sized (1,000 micron) water drop in a specially designed chamber. This step puts the droplet at a temperature below its freezing point but leaves it free of ice crystals, thus ensuring the same starting conditions for all experiments. The researchers then set the freezing process in motion by touching the drop with a tip.
The team's high-speed videos reveal that the freezing process in drops is complex. Within a few microseconds of being touched, a “shell” of solid ice encapsulates the drop and starts to thicken inwards, compressing still-liquid water. Some of the building pressure is released by an “arm” of ice that extends from one side of the drop. But eventually, cracks and bubbles form, and within about two seconds of the process beginning, the droplet shatters. Turning to their model, the group predicts that drops with diameters larger than 50 μm will always explode when frozen because of their high inner pressures. Smaller drops, however, never burst because the surface tension of the shell is strong enough to keep them intact.